The injectors used in fuel injection systems are generally controlled electrically by means of a current waveform applied to the injector. The properties or shape of the waveform applied to the injectors determines the type of injection performed by the injectors. For example, a first waveform may be arranged to cause the injector to generate a pilot injection followed by a single main injection while a second waveform may be arranged to generate a single main injection with no preceding pilot injection.
In order to optimise the operation of the injectors, the waveform must be arranged to start and end at the correct time within the injection cycle. The start and end times for each type of waveform will generally vary in dependence on the instantaneous operating condition of the engine and in particular on the engine speed and the fuel demand or engine load. Moreover, the start and end times for a given operating condition may be different for each type of injection cycle.
Values representing the start time and the duration of the waveform, the latter effectively defining, in conjunction with the start time, the end time for the waveform, are called or calculated by means of one or more maps stored in a memory within the engine controller or management system.
Each map generally comprises a two-dimensional table having ordinate and abscissa values representative of fuel demand (engine load) and engine speed. Each point in the table is an output value representative of a start time for the waveform for a given combination of engine speed and load (hereafter referred to as an engine “condition”). For an engine condition which does not correspond to a discrete point in the table, an output value is derived by interpolating from surrounding points in the table. The interpolated output value is used by an algorithm to generate the appropriate current waveform with the correct start time. A similar table is used to derive the required duration of the waveform, thereby to define the correct end time for the waveform.
A problem with the above-described system is that, owing to the complexity of modern injectors and their ability to perform more than one injection or part injection per cycle, the use of different types of injection cycle (i.e. different combinations of injections or part injections) during certain parts of the engine operating envelope means that at least a pair of maps (one for calculating the start time of the wave form and one for calculating its duration) is required for each type of injection cycle. This is highly wasteful of the memory within the engine management system or controller.
The present invention seeks to address the above problem.